1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and a semiconductor device, and more specifically, it relates to a method of fabricating a semiconductor device capable of forming a precision fine pattern not implementable solely by photolithography and a semiconductor device.
2. Description of the Prior Art
Following improvement in the degree of integration of a semiconductor device such as a DRAM (dynamic random access memory), a fine pattern having a size smaller than the wavelength of a light source employed for photolithography must be formed on a semiconductor substrate. Such a fine pattern can be formed by a method of preparing a gate electrode from polycrystalline silicon (hereinafter referred to as polysilicon).
This method is now described with reference to FIGS. 23 to 27. Referring to FIG. 23, a gate insulator film 113 is formed on a silicon substrate 101, and a polysilicon film 108 is deposited thereon. A silicon oxide film 109 is arranged on the polysilicon film 108, and a photoresist pattern 111 is formed thereon. This photoresist pattern 111 is employed as a mask for dryly etching the silicon oxide film 109 as shown in FIG. 24. A pattern 109a obtained from the silicon oxide film 109 is dipped in a chemical solution containing hydrofluoric acid, to be thinned. In other words, the width of the pattern 109a is narrowed for obtaining a precision-made pattern 109b (FIG. 25). Thereafter the precision-made pattern 109b is employed as a mask for dryly etching the polysilicon film 108. Consequently, a polysilicon pattern 108a for defining a gate electrode is obtained (FIG. 26).
Silicon oxide films are deposited on the aforementioned polysilicon pattern 108a for forming side wall spacers 114. Consequently, a gate electrode having a small width can be formed (FIG. 27).
In order to deposit cobalt on the polysilicon pattern 108a for forming an electrode of cobalt silicide through a salicide process, however, the silicon oxide film 109b employed as the mask must be removed with hydrofluoric acid or the like. When the silicon oxide film 109b is removed with hydrofluoric acid or the like, however, the gate oxide film 113 and the patterns of the silicon oxide films such as the side wall spacers 114 around the gate electrode are also etched at the same time. Such etching is disadvantageous. Thus, awaited is a method of forming a fine pattern not implementable solely by photolithography with no pattern shifting etching unnecessary portions or the like.
When a MOS transistor is fabricated in practice, a silicon oxynitride film 112 may be deposited between the silicon oxide film 109 and the photoresist pattern 111 as an antireflection film for photolithography, as shown in FIG. 28. In order to etch the polysilicon film 108 through the pattern 109b of the silicon oxide film 109 thinned by wet etching, this antireflection film 112 must be removed before the wet etching. If the antireflection film 112 is removed by wet etching employing phosphoric acid or the like, however, the silicon oxide film 109 for forming the mask 109b is also etched. Therefore, the size of the finally obtained polysilicon pattern 108a disadvantageously fluctuates.
Further, the pattern 109b of the silicon oxide film 109 employed as the mask for dry etching as described above must be finally removed. However, a chemical solution employed for removing the pattern 109b of the silicon oxide film 109 by wet etching inevitably etches the remaining silicon oxide films forming the peripheral portions and the silicon oxynitride film 112. When the mask pattern 109b of the silicon oxide film 109 is removed, therefore, the gate electrode and a portion around a contact hole are also etched to deteriorate dimensional accuracy.
An object of the present invention is to provide a method of fabricating a semiconductor device having a fine structure not implementable solely by photolithography with no pattern shifting on a peripheral oxide film or when removing an antireflection film as well as a mask pattern, and the semiconductor device.
The method of fabricating a semiconductor device according to the present invention comprises steps of forming a base film of either a silicon film or a silicon compound film on a semiconductor substrate and forming a hard film of either a metal film or a metal compound film in contact with the upper portion of the base film. The method also comprises steps of forming a photoresist pattern in contact with the upper portion of the hard film and dryly etching the hard film through the photoresist pattern serving as a mask for forming a hard pattern. The method further comprises steps of dryly etching the base film through the hard pattern serving as a mask and removing the hard pattern by wet etching with a chemical solution not substantially etching at least the base film.
According to this structure, the hard film is formed by a metal-based film, and the base film is formed by a film mainly composed of silicon. In the step of removing the hard pattern by wet etching, therefore, the chemical not etching the pattern of the base film can be selected with no significant difficulty. The above wording xe2x80x9cchemical solution not substantially etching at least the base film (but etching the hard film)xe2x80x9d stands for a chemical solution having large etch selectivity for the hard film with respect to the base film. When the base film is formed by a polysilicon film and the hard film is formed by a metal film of tungsten or the like, for example, a chemical solution containing an oxidant such as hydrogen peroxide water or ozone water corresponds to this chemical solution.
Thus, the pattern of the base film formed by dry etching is not influenced by the aforementioned chemical solution. Therefore, neither a gate electrode nor a contact hole causes pattern shifting in the aforementioned wet etching for removing the hard pattern. Thus, a semiconductor device having a fine precision structure can be obtained. Consequently, the semiconductor device having a fine structure can be fabricated with an excellent yield. The base film may be in contact with the upper portion of the semiconductor substrate, or may be formed on another film interposed between the same and the semiconductor substrate.
The aforementioned method of fabricating a semiconductor device according to the present invention further comprises a step of wetly etching the hard pattern with a chemical solution substantially not etching at least the base film for forming a precision-made hard pattern between the step of forming the hard pattern and the step of dryly etching the base film. In the step of dryly etching the base film, the base film can be dryly etched through the precision-made hard pattern serving as a mask.
According to this structure, a precision hard pattern not implementable solely by photolithography can be obtained without exerting bad influence such as pattern shifting on at least the base film or the remaining portions. Further, the hard pattern can be removed without exerting bad influence on at least the base film or the remaining portions as hereinabove described.
When this precision-made hard pattern is employed, a fine gate electrode and a precision contact hole can be formed in a refined MOS transistor.
The aforementioned method of fabricating a semiconductor device according to the present invention further comprise a step of removing the photoresist pattern after the step of forming the hard pattern by dry etching.
When the base film is dryly etched through the mask of the aforementioned hard pattern, the photoresist pattern may remain in contact with the upper portion of the hard film. However, precision may be more readily improved if the photoresist pattern is removed. Therefore, the photoresist pattern is preferably removed by ashing or the like before the step of etching the base film through the mask of the hard pattern. When the hard pattern is wetly etched for forming the precision-made hard pattern, the aforementioned photoresist pattern is removed by ashing or the like generally before the step of forming the precision-made hard pattern.
In the aforementioned method of fabricating a semiconductor device according to the present invention, processing is performed while leaving the photoresist pattern intact after the step of forming the hard pattern by dry etching. In the step of removing the hard pattern, the photoresist pattern can be removed by wet etching along with the hard pattern.
According to this structure, the base film can be dryly etched through the mask of the hard pattern while leaving the photoresist pattern intact. In this case, the hard pattern may be improved in precision by wet etching while holding the photoresist pattern. In the method of fabricating a semiconductor device according to the present invention, the hard pattern is wetly etched while leaving the photoresist pattern located in contact with the upper portion thereof intact in the wet etching step for improving the precision of the hard pattern. The precision-made hard pattern having the photoresist pattern located thereon is employed as a mask for dryly etching the base film. Then, the photoresist pattern can be removed by wet etching along with the (precision-made) hard pattern in the step of removing the hard pattern.
The chemical solution employed for wetly etching the aforementioned hard pattern generally also etches the photoresist pattern. Thus, the photoresist pattern is also wetly etched along with the hard pattern in the wet etching step for forming the aforementioned precision-made hard pattern. Also when the photoresist pattern is held, therefore, the hard pattern can be improved in precision with no problem. When the hard pattern and the photoresist pattern are removed, the chemical solution does not etch at least a peripheral portion such as the base film to result in pattern shifting, as described above. Thus, a semiconductor device having a fine structure and excellent dimensional accuracy can be obtained.
In the aforementioned method of fabricating a semiconductor device according to the present invention, the hard film can consist of a film containing at least one of titanium, titanium nitride, tungsten and tungsten nitride.
When the hard film is prepared from titanium, tungsten or a compound thereof as described above, a readily purchased raw material having excellent circulativity can be employed as a proper mask material for dry etching. Further, a chemical solution having large selectivity with respect to the hard film and the base film can be extremely readily selected in wet etching.
In the aforementioned method of fabricating a semiconductor device according to the present invention, the base film can be any of a silicon film, a silicon oxide film, a silicon nitride film, a silicon oxynitride film and a metal silicide film.
The aforementioned material for the base film, facilitating film formation or the like when the semiconductor device is formed on a silicon substrate, is extremely frequently employed for fabricating a semiconductor device of silicon. When the hard film is formed by a metal film and the base film is made of the aforementioned material, the semiconductor device can be efficiently fabricated at a low cost without employing an extremely specific chemical solution. Further, heterogeneity of etched tendencies of the metal film and the aforementioned base film can be increased with a relatively general chemical solution, whereby the range for selecting an etching solution having large selectivity can be widened. The silicon film may contain a p-conductivity type or n-conductivity type impurity, and a polysilicon film, an amorphous silicon film or an epitaxial silicon film (single-crystalline silicon film) corresponds thereto. CoSi, TiSi, WSi or the like corresponds to the metal silicide in chemical formula expression.
In the aforementioned method of fabricating a semiconductor device according to the present invention, a chemical solution containing an oxidant can be employed in at least either the step of removing the hard pattern by wet etching or the step of forming the precision-made hard pattern by wet etching.
In the aforementioned method of fabricating a semiconductor device according to the present invention, a chemical solution containing acid can be employed in at least either the step of removing the hard pattern by wet etching or the step of forming the precision-made hard pattern by wet etching.
Alternatively, a chemical solution containing ammonia can be employed in at least either the step of removing the hard pattern by wet etching or the step of forming the precision-made hard pattern by wet etching.
When the semiconductor substrate is formed by a silicon substrate, it is highly probable that the material for the base film is a polysilicon film or a silicon oxide film mainly composed of silicon as described above. In the point of the aforementioned high etch selectivity, the metal film is utterly different from the aforementioned material for the base film. An etching solution having a high etch rate for the metal film and a low etch rate for silicon or the like may be prepared from a chemical solution containing an oxidant, a chemical solution containing acid or a chemical solution containing ammonia. The oxidant may be prepared from hydrogen peroxide water or ozone water, for example.
Such a chemical solution is properly selected in response to the type of the metal forming the hard film. For example, ammonia is relatively properly employed when the hard film is made of tungsten or tungsten nitride, hydrofluoric acid is relatively preferably employed when the hard film is made of titanium or titanium nitride.
A precision fine pattern not implementable by photolithography can be obtained under prescribed conditions by employing the aforementioned chemical solution containing an oxidant, acid or ammonia.
In the aforementioned method of fabricating a semiconductor device according to the present invention, the hard pattern can be removed with at least one of a chemical solution containing hydrogen peroxide, a chemical solution containing ozone water, a chemical solution containing sulfuric acid, a chemical solution containing hydrochloric acid, a chemical solution containing phosphoric acid, a chemical solution containing nitric acid, a chemical solution containing acetic acid, a chemical solution containing hydrofluoric acid and a chemical solution containing ammonia in the step of removing the hard pattern by wet etching.
Each of the aforementioned chemical solutions has large etch selectivity with respect to a metal film or a metal compound film of Ti, W, TiN or WN, a silicon film, a silicon oxide film, a silicon nitride film and a metal silicide film. In the step of removing the hard pattern or the precision-made hard pattern, therefore, the hard pattern can be removed without substantially etching the base film. Also when the hard pattern or the precision-made hard pattern has a photoresist pattern, the hard pattern is removed along with the photoresist pattern, not to exert bad influence on other portions.
In the aforementioned method of fabricating a semiconductor device according to the present invention, the hard pattern can be removed with at least one of a chemical solution containing hydrogen peroxide, a chemical solution containing ozone water, a chemical solution containing sulfuric acid, a chemical solution containing hydrochloric acid, a chemical solution containing phosphoric acid, a chemical solution containing nitric acid, a chemical solution containing acetic acid, a chemical solution containing hydrofluoric acid and a chemical solution containing ammonia in the step of wetly etching the hard pattern for forming the precision-made hard pattern.
A semiconductor device according to the present invention, formed on a semiconductor substrate, is fabricated by the aforementioned method of fabricating a semiconductor device.
According to this structure, the inventive semiconductor device can have a precision fine pattern. Thus, the yield is so improved that a semiconductor device having high reliability can be obtained at a low cost.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.